To improve an efficiency of an engine at the time of its low load operation or high-speed rotation.

A sliding valve main body 6 is arranged within a sub-cylinder 2, and a maximum cavity 7 is arranged within the sliding valve main body 6. There is provided a driving device for changing-over communication and separation between a main combustion chamber and the maximum cavity 7. In the case that the communication between the maximum cavity 7 and the main combustion chamber and their separation are to be changed over in accordance with a load, the sliding valve may function as a variable compression ratio device and then a third compression ring is provided. In the case that the communication between a sub-flow passage and the main combustion chamber and their separation are changed over when an intake stroke or an exhaust stroke is carried out, the sliding valve may function as an intake valve or an exhaust valve so as to improve an intake and exhaust efficiency.